Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 313.9	$ 325.2	$ 523.0	$ 530.7
Net income allocated to participating securities	(3.9)	(6.3)	(6.7)	(10.4)
Net income available for common shares	$ 310.0	$ 318.9	$ 516.3	$ 520.3
Weighted Average Shares:
Basic	244.5	256.2	245.5	257.7
Dilutive stock options and restricted shares	1.2	2.0	1.1	2.1
Diluted	245.7	258.2	246.6	259.8
Anti-dilutive stock options and restricted shares	0.1	0.1	0.1	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.27	$ 1.24	$ 2.10	$ 2.02
Diluted	$ 1.26	$ 1.23	$ 2.09	$ 2.00